In recent years, a thermal recording system in which recording is carried out by heat energy is employed in many cases of various kinds of information appliances such as a facsimile machine, a printer, a recording equipment, and so on. The thermal recording material used in the thermal recording system has many excellent characteristics that a degree of whiteness is high, appearance or touch feeling is close to plain paper, recording suitability such as thermal coloring sensitivity and so on is good, and so on. In particular, a material which uses a colorless or pale colored electron donating dye precursor (in particular, a leuco dye) as a coloring agent and uses an acidic developer such as a phenolic compound has a characteristic that reactivity of the dye precursor which is an electron donating compound is high. There is an advantage that a colored image with high density can be obtained instantaneously when the dye precursor is contacted with the developer which is an electron accepting compound. On the other hand, it has drawbacks of poor storage stability of the recording that the obtained colored image is inferior in chemical resistance, so that the recording is likely disappeared by contacting with a plasticizer contained in a plastic sheet or an eraser or a chemical contained in foods or cosmetics, or the recording part is poor in light resistance, so that the recording is faded or disappeared by sunlight exposure with a relatively short period of time, and so on.
Thus, it has been proposed to use a mixture of 2,4′-dihydroxydiphenylsulfone and 4,4′-dihydroxydiphenylsulfone with various weight ratio as a developer which can relatively improve storage stability of recording (for example, see Patent Documents 4 and 5). Among these, 2,4′-dihydroxydiphenylsulfone has excellent basic properties as a thermal recording material, such as thermal coloring sensitivity, a degree of whiteness, and so on, so that there are disclosed inventions that the weight ratio of 2,4′-dihydroxydiphenylsulfone is heightened to be used for a thermal recording material (for example, see Patent Documents 1 and 6). However, 2,4′-dihydroxydiphenylsulfone is relatively inferior in recording stability, and production thereof is relatively difficult, and it has low production with high manufacturing cost. On the other hand, 4,4′-dihydroxydiphenylsulfone is relatively low in properties as a thermal recording material but its recording stability is good. In addition, it has conventionally been used as a starting material of polysulfone and so on, so that production thereof is relatively easy and it has been mass-produced with low manufacturing cost. By this reason, there have been disclosed inventions that a weight ratio of 4,4′-dihydroxydiphenylsulfone is heightened to be used for a thermal recording material (for example, see Patent Documents 2 and 3).
These 2,4′-dihydroxydiphenylsulfone and 4,4′-dihydroxydiphenylsulfone used in the thermal recording material have heretofore been manufactured as a mixture of isomers, by the manufacturing method in which one step reaction process is carried out by reacting a sulfonating agent, phenolsulfonic acid or sulfuric acid, and so on, with phenol. An original object of this manufacturing method is to manufacture the 4,4′-dihydroxydiphenylsulfone which is a starting material for a polysulfone and so on. According to this manufacturing method, the 4,4′-dihydroxydiphenylsulfone is formed with a weight ratio of, for example, 80 to 99.9%, and 2,4′-dihydroxydiphenylsulfone which is a byproduct is formed with a weight ratio of, for example, 0.1 to 20%. For manufacturing efficiently the 4,4′-dihydroxydiphenylsulfone which is the original objective material of this manufacturing method, it is usual to optimize the settings of the respective conditions of the reaction and so on, so that the weight ratio of the 2,4′-dihydroxydiphenylsulfone which is a byproduct becomes as small as possible.
However, in the case of the 2,4′-dihydroxydiphenylsulfone with larger weight ratio to use for the purpose of improving various properties of the thermal recording material, it is necessary to undergo the procedure that the 2,4′-dihydroxydiphenylsulfone or the 4,4′-dihydroxydiphenylsulfone is separated from the dihydroxydiphenylsulfone mixture formed by the above-mentioned manufacturing method, and then, the separated compound is added and mixed and so on, so that the resulting mixture has an objective weight ratio. This manufacturing process is shown in FIG. 2. The dihydroxydiphenylsulfone mixture thus manufactured has heretofore been used for a thermal recording material.
However, the 2,4′-dihydroxydiphenylsulfone and the 4,4′-dihydroxydiphenylsulfone have properties that solubilities thereof to a solvent for separation from each other are very close. Therefore, it is necessary for each purity improvement to undergo complex separating steps such that a pH adjustment and so on is carried out and a separating step is repeated. In order to obtain the 2,4′-dihydroxydiphenylsulfone with a high concentration, particularly in the usual case where the reaction conditions are previously so set that the formation ratio at the reaction of the 2,4′-dihydroxydiphenylsulfone becomes as small as possible, it is necessary to carry out the procedure to increase the concentration by using the mixture having the weight ratio only 0.1 to 0.2% for example. This procedure requires significant labor and time, whereby it causes high manufacturing cost of a thermal recording material which uses the 2,4′-dihydroxydiphenylsulfone with a high weight ratio.
Also, it has been known that in this manufacturing method, phenyl phenol-sulfonate or trihydroxytriphenyldisulfone is formed as an impurity, and contained in the mixture. Further, it has been also known that these impurities cause bad effects on thermal coloring sensitivity and storage stability of the background of the thermal recording material (Patent Document 5, Patent Document 6).
By the way, there is disclosed an invention that 4,4′-dichlorodiphenylsulfone is produced by using starting materials such as dialkyl sulfate and chlorobenzene, and reacting them for the purpose of obtaining a starting material of 4,4′-diaminodiphenyl-sulfone which becomes a curing agent of an epoxy resin. There are also disclosed that 2,4′-dichlorodiphenylsulfone is synthesized as a byproduct in the reaction, and the resulting product becomes a mixture of isomers, and further, the mixture of isomers is hydrolyzed to form a mixture of 4,4′-dihydroxydiphenylsulfone and 2,4′-dihydroxydiphenylsulfone by the two steps reaction (see Patent Document 7).
However, according to this manufacturing method, a dichlorodiphenylsulfone mixture is synthesized through the steps firstly by the reaction using the commercially available starting materials, and then hydrolysis of the resulting product to obtain a dihydroxydiphenylsulfone mixture. That is, it is necessary to undergo at least two steps of the reaction procedure of the synthesis and the hydrolysis. Therefore, the manufacturing method which undergoes the two steps of the reaction procedure has been considered to be a disadvantageous method since the manufacturing cost is clearly high as compared with the conventional manufacturing method which is to obtain a dihydroxydiphenylsulfone mixture directly by the one step of the reaction procedure from the commercially available starting materials. Accordingly, the manufacturing method of Patent Document 7 has not heretofore been considered to be a realistic method for obtaining the dihydroxydiphenylsulfone mixture.